Which of the following protects life on earth from harmful effects of $UV$ radiations from the sun?

Fill in the blanks:
$(1)$ Hydrocarbon compounds are made from ........ and ........ .
$(2)$ ........ has a capacity to stop the flow of oxygen.
$(3)$ ........ is formed by attaching carbon monoxide with hemoglobin in blood.

Which of the following is a secondary pollutant?

The possibility of photochemical smog formation will be minimum at

Some statements are given:
$(S_1) \ CO_2$ is a greenhouse gas.
$(S_2)$ Classical smog contains smoke,fog,and a high concentration of $SO_2$.
$(S_3) \ DDT$ is biodegradable.
$(S_4)$ Drinking water should have a high $B.O.D.$
Which is the correct sequence of true $(T)$ or false $(F)$ regarding the above statements?

Air pollution that occurs in sunlight is